In a furnace using a so-called two-stage combustion in which a fuel is burned by burners under a condition of air deficiency, and the remaining air required for complete combustion is supplied from after-air ports, a flow rate distribution of combustion gas containing unburned components rising to an after-air port region varies according to an arrangement of the burners and a method of supplying the fuel and air from the burners. To suppress the unburned components such as unburned carbon or CO remaining in the furnace outlet, it is important to appropriately supply the two-stage combustion air depending on the flow rate distribution of the combustion gas rising to the after-air port region.
FIG. 14 is a view illustrating an example of an arrangement of burners 6, after-air ports 7a, sub after-air ports 7b and shapes of jets in the furnace in the related art. FIG. 14(a) is a front view illustrating a furnace wall in which the burners 6, the after-air ports 7a and the sub after-air ports 7b are disposed, FIG. 14(b) is a view (side sectional view) illustrating shapes of jets consisting of fuel and air injected from the burners 6, the after-air ports 7a and the sub after-air ports 7b as viewed from a side surface of the furnace, and FIG. 14(c) is a plan sectional view of the furnace illustrating the shapes of after-air jets as viewed from the top, which is a view taken in an arrow direction of line B-B in FIG. 14(b).
In the furnace illustrated in FIG. 14, the burners 6 are disposed to the both opposed faces in four rows and three stages, the after-air ports 7a are installed above the burners 6, and the sub after-air ports 7b are installed nearer furnace side walls at a slightly lower height than the height of the after-air ports 7a. The fuel and air injected from the burners 6, the after-air ports 7a and the sub after-air ports 7b which are installed on opposed furnace front and rear walls collide at the central part of the furnace in a depth direction (anteroposterior direction) thereof, as illustrated in FIGS. 14(b) and 14(c), and after colliding, mainly flow toward an upper side, as illustrated in FIG. 14(b). As a result of the above-described flow pattern in the furnace, the flow rate distribution of the rising gas at a central part in the furnace depth direction just below the after-air port region on an A-A line cross-section of FIG. 14(b) becomes a form illustrated by a solid line in FIG. 15(a), and the flow rate distribution of the rising gas at the central part in the furnace width direction on the same A-A line cross-section becomes a form illustrated by the solid line in FIG. 15(b).
The jets of the fuel and air from the burners 6 disposed on the opposed front and rear walls as illustrated in FIG. 14(a) collide at the central part in the furnace depth direction to change the direction thereof, but the flow toward the upper side which is the gas outlet side of the furnace becomes greatest, such that, as illustrated by the solid line in FIG. 15(a), the flow rate is highest just above the burner row, while the flow rate is lower between the burner rows and between the wing burner rows and the side walls. As a result of the flow in the furnace, in the flow rate distribution as viewed from the central part in the furnace width direction from the side wall side (FIG. 15(b)), it becomes a distribution that the flow rate is highest at the central part in the furnace depth direction, while the flow rate is lower in the vicinity of the front and rear walls of the furnace.
If the above-described flow rate distribution of the rising gas in the furnace is broadly classified, it may be divided into a region A (a portion surrounded by a dotted line frame in FIGS. 15(a) and 15(b)) having relatively high flow rates in the vicinity of the central part of the furnace depth and width directions, regions C (portions surrounded by a one-dot dash line frame in FIG. 15(b)) having relatively low flow rates at the front and rear walls, and regions B (portions surrounded by a two-dot dash line frame in FIG. 15(a)) having relatively low flow rates in the vicinity of the side walls. In order to minimize the unburned components remaining at the furnace outlet, it is important that after-air having an appropriate flow rate and appropriate momentum is supplied to all the regions A, B and C from the after-air ports 7a and 7b, to facilitate the mixing in an appropriate ratio of the unburned components and the air at the respective regions A, B and C.
Patent Literature 1 (Japanese Unexamined Patent Application Publication No. 2007-192452) discloses a boiler device which is characterized in that, in a combustion device for a solid fuel such as coal, a direction of after-air blowing out into a furnace from after-air ports is horizontally divided into three or more directions; and an air dividing member is provided therein, so that the respective divided directions of air do not become the same direction as each other.
Patent Literature 2 (Japanese Patent No. 5028278) discloses an invention of a pulverized coal-fired boiler including: a furnace which forms the pulverized coal-fired boiler; a plurality of burners disposed on an upstream side of a furnace wall surface to supply pulverized coal of fuel and air into the furnace and to burn the same; and a plurality of after-air ports disposed on the furnace wall surface which is to be an upper side from a position in which the burners are installed to supply the air, wherein the after-air ports consist of main after-air ports supplying a large amount of air and sub after-air ports supplying a small amount of air.
The invention described in Patent Literature 2 is the pulverized coal-fired boiler in which: the sub after-air ports are disposed on the furnace wall surfaces which is to be a downstream side of the main after-air ports and at a position of the furnace wall surface just above the main after-air ports, or disposed on the furnace wall surfaces which is to be the upstream side of the main after-air ports and at a position of the furnace wall surface just below the main after-air ports; a sectional center of each of the sub after-air ports is within a range of 1 time or more to 5 times or less of a diameter of the main after-air ports from a sectional center of the main after-air ports, one main after-air port and one sub after-air port are set to be one pair, and at least one pair is connected to the same wind box; and a plurality of the wind boxes are installed by arranging on the furnace wall surface in one direction.
Patent Literature 3 (Japanese Unexamined Patent Application Publication No. S58-224205) discloses a combustion device having OA ports configured to perform two-stage combustion or denitration combustion in the furnace, wherein the combustion device includes: a combustion method, in which small sub OA ports are disposed nearer the side walls than the row of wing burners to improve the supply of the air to the vicinity of the side walls, so as to more sufficiently exert the function of the OA ports performing a complete combustion; and a method for reducing unburned components at a furnace outlet which is capable of controlling a direction of an airflow by mean of swirl generation of the OA ports.
It is effective to adopt a configuration including the auxiliary OA ports of Patent Literature 3 as a means for appropriately supplying two-stage combustion air in the vicinity of the side walls of the regions B illustrated by the two-dot dash line frame in FIG. 15.
As a method of supplying air to the regions B in the vicinity of the side walls of the furnace, it may be supplied from openings installed in front and rear walls in the vicinity of the side walls as the invention described in Patent Literature 3, and it may be supplied from one or more openings installed in the side walls. In addition, there is a case in which the air flow rate supplied from the burners and after-air ports near the side walls is higher than the air flow rate supplied from the burners and the after-air ports positioned at the central side in chamber width (furnace full width) direction, such that the air flow nearer the side walls is increased, and thereby a similar effect of reducing the unburned components is obtained.
Patent Literature 4 (Japanese Unexamined Patent Application Publication No. 2001-355832) discloses a configuration including: a cylindrical sleeve which is provided to divide an air flow passage in an air port; and a baffle which is attached to a tip of the sleeve at the exit of the sleeve so as to spread the flow in the air flow passage to the outside from a center axis of the air port, wherein a spreading part of the sleeve and the baffle have the same inclination angle as each other. This is an invention in which, due to the above-described configuration, it is possible to spread the airflow by the inclination angle of the spreading part of the sleeve and the tip of the baffle without a swirl generating device, and increase a mixing rate with a combustion gas from the burner on the upstream side of the air ports.
Patent Literature 5 (US Patent Publication No. 2012/174837) describes a configuration which is capable of changing a direction of the flow of after-air within a furnace by providing vanes which can change a flow direction of the air at an outlet in an air port.
Patent Literature 6 (Japanese Patent Publication No. 2717959) discloses a multi-directional control device for an after-air hole of a type which has an after-air hole configured to send secondary air from an opening of a wind box to an opening of a furnace, and a longitudinal conduit for defining a chamber, wherein the secondary air from the wind box passes through the chamber toward the furnace. In addition, the multi-directional control device disclosed in the above document includes a plurality of first louvers which are rotatably mounted inside of the chamber with respect to the conduit based on a first axis orthogonal to a longitudinal axis of the conduit, a plurality of second louvers which are rotatably mounted inside of the chamber with respect to the conduit based on a second axis orthogonal to the longitudinal axis of the conduit and orthogonal to the first louver, and a means configured to control an air flow direction passing through the opening of the furnace by rotating each of the first louver and the second louver.